1. Field of the Invention
The present invention relates to a light emitting device, a wafer for a light emitting device, and a method of preparing the same, and more specifically, it relates to a light emitting device looking blue or blue-green, a wafer for a light emitting device and a method of preparing the same.
2. Description of the Background Art
In recent years, GaN light emitting devices are watched with interest. Among such light emitting devices, a device employing a sapphire substrate is now being industrialized in particular. However, it is difficult to form a metal electrode on a lower portion of the sapphire substrate which is an insulating substrate. Therefore, a nitride mixed crystal must be extracted from an upper portion of the sapphire substrate by etching for forming a metal electrode, leading to complicated preparation steps and a high cost.
Due to there being no cleavage of the sapphire substrate, further, a reflector consisting of a cleavage plane cannot be formed in application to a laser diode. To this end, there has generally been proposed a light emitting device employing a gallium arsenide (GaAs) substrate in place of the sapphire substrate having such disadvantages. For example, Japanese Patent Laying-Open No. 8-181070 (1996) discloses such a light emitting device.
However, such a light emitting device employing a GaAs substrate has large absorption for light of a shorter wavelength corresponding to blue. Therefore, light passing through the GaAs substrate is absorbed to disadvantageously reduce the luminance of the overall light emitting device. In general, factors deciding the luminance of a light emitting device include light extraction efficiency in the case of extracting light from a light extraction port of the light emitting device into the air. This light extraction efficiency is decided by factors based on light reflection in the crystal and absorption of the light into the crystal. When a GaAs substrate is employed, however, a light absorption coefficient (cm.sup.-1) which is the latter factor exceeds 10,000 with respect to light having energy exceeding 1.48 eV, i.e., in excess of 100 times that of a general sapphire substrate. Therefore, the GaAs substrate disadvantageously presents larger absorption as compared with the sapphire substrate.